As is known, multiple-arm measuring machines comprise two or more measuring units, each with its own measuring tool, which operate in coordination under the control of a common control system. The measuring units are normally positioned with their respective measuring volumes side by side and overlapping at a small intersection, so the overall measuring volume of the machine is defined by the combined measuring volumes of the individual units. Multiple-arm measuring machines of the above type are therefore particularly suitable for measuring large-size parts, such as vehicle bodies or aircraft components.
In a typical embodiment, to which the following description refers for convenience and purely by way of example, the machine comprises two horizontal-arm cartesian measuring units located on opposite sides of the measuring volume, and each unit comprises a column movable along a longitudinal first axis with respect to the measuring volume, a carriage fitted to the column and movable along a vertical second axis, and an arm fitted to the carriage and movable with respect to it along a horizontal third axis perpendicular to the first axis and crosswise to the measuring volume.
In multiple-arm machines employing coordinate measuring units (particularly machines with two horizontal arms), aligning the cartesian reference system of one of the two arms (the secondary or “slave” arm) with respect to the other (the “primary” or “master” arm) is vital to measuring performance in two-arm mode.
The usual alignment method comprises measuring a sphere variously positioned at the intersection between the measuring volumes of the two units, and accordingly rotating and translating the cartesian reference system of the secondary arm with respect to that of the primary arm.
Measuring machine performance in multiple-arm mode depends closely on the compensation precision and dimensional stability of the individual units, and on the precision and stability of the results of the above alignment procedure.
The latter, in particular, is affected by deformation of both measuring units caused by variations in ambient temperature, which may result in distortion of the geometry of both units, not entirely recoverable by the geometric compensation procedure, and in elongation of the component parts of the units (transducers, beams, etc.), which often results in measuring errors serious enough to impair performance.
Distortion of individual units also results in even serious measuring errors in multiple-arm mode.
Frequently updating alignment of the cartesian reference systems of each unit of a multiple-arm machine is therefore of vital importance, but in actual fact difficult, if not impossible, to do in the case of on-line measuring systems, which rule out a fixed floor-mounted sphere for obvious accommodation reasons.
Another factor affecting the performance of multiple-arm machines is the weight of the workpiece, which may cause significant yield of the foundation and/or bed on which the units are installed, thus affecting the no-load-determined alignment conditions of the units.
One way of minimizing this effect is to align the systems with the workpiece set up in place, though often the very size of the workpiece prevents this. A “mockup” is another possible solution, but often unpractical and technically unfeasible, by involving additional movements, possibly interfering with dedicated workpiece supporting fixtures, and possibly differing considerably from the load configuration of the actual workpiece. The problem with this solution is further compounded by the load varying with different workpieces.
The only solution to these problems lies in oversizing the foundation and/or bed, thus increasing the cost of the machine.